See Help:Range blocks for the main page on rangeblocking, and for IPv4. Like IPv4, IPv6 rangeblocks are limited by $wgBlockCIDRLimit, which by default allows rangeblocks of up to /64 in size (before MediaWiki 1.20wmf5, which changes the default to /19).

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Technical explanation

IPv6 addresses are each 128 bits long. Because each digit in an IPv6 address can have 16 different values (from 0 to 15), each digit represents the overall value of 4 bits (one nibble), with 32 digits total. As with IPv4, CIDR notation describes ranges in terms of a common prefix of bits. For example 2001:db8::/32 means that the range described has the first 32 bits set to the binary digits 00100000000000010000110110101000. Also like IPv4, MediaWiki implements IPv6 rangeblocks using CIDR notation.

Collateral damage

The best guideline for assessing collateral damage for an IPv6 rangeblock is to check the WHOIS, and the actual amount of activity from the range. In wikis with Gadget-contribsrange.js gadget available this can can be done with a wildcard search on user contributions; the gadget, if available, may need to be enabled in user preferences. For example 2001:DB8:0:0:* would search for contributions in a 2001:db8::/64 range; search terms must be in capitals with no unneeded preceding zeroes.

A minimum /64 subnet for a connection is a practical requirement of IPv6. IPv6 requires minimum /64 subnetting as any deviation from this breaks a great number of IPv6 protocols; and it will also be needed for future developments. See IPv6 Unicast Address Assignment Considerations for a discussion on this topic. The only exception to larger than /64 subnetting is for certain connections where many IPv6 features are not needed. For example inter-router linking may use this for limited uses; see Using 127-Bit IPv6 Prefixes on Inter-Router Links for a discussion on this. But this is not relevent for IPv6 rangeblocking.

But like in IPv4, where a single static IPv4 address may supply a household or institution of some kind, a static IPv6 /64 range may also supply a household or institution, so multiple users can be affected by /64 rangeblocks.

How to calculate an IPv6 CIDR range

Although similar principles also apply to IPv4, the fact that hexadecimal digits correspond to exactly one nibble (4 bits) each simplifies the calculation somewhat.

Find the common prefix - that is, the string of digits shared by all of your addresses.

Expand the common prefix completely (including redundant zeros), and multiply the total number of digits by 4 - that's the size of your common prefix.

Use the table below, and add the specified number to your prefix size (this is your CIDR suffix, the number of bits common to all addresses in the range). Append the specified digit to the common prefix you found above. Complete the group of four digits with zeros if necessary.

The CIDR is then [common prefix (with additional digit as specified by table below, followed by zeros as necessary to keep the group of four digits complete)]/[common prefix length (plus 1,2, or 3 as specified by the table below)].

Prefix modification table

Example: If your prefix is 2001:db8:abcd:: (completely expanded to 2001:0db8:abcd::), and you have addresses like 2001:db8:abcd:9000:abe:: and 2001:db8:abcd:a000::, you'd use the entry saying that the digits after your prefix match 8-b, append 8 to the prefix, and add 2 to your common prefix's size. This would generate 2001:db8:abcd:8000::/50.